1. Field of the Invention
The present invention concerns a bipolar transistor. The present invention is useful when applied, for example, to a bipolar transistor using silicon/germanium as a base layer. More in detail, the present invention concerns a bipolar transistor which operates with high accuracy, and which is suitable for high speed operation at high gain.
2. Related Art
For increasing the operation speed of bipolar transistors, it is necessary to reduce the thickness of the base layer. In this case, if the thickness of the base layer is simply decreased, the result is a problem of depletion of the base layer, which causes a short circuit between the collector and the emitter when voltage is applied between the collector and the emitter. As the concentration in the base layer is generally increased along with reduction of the thickness of the base layer, this decreases a collector current which results in a problem that no sufficient current gain can be obtained. On the other hand, since a bipolar transistor using single crystal silicon/germanium as a base layer has no such a problem, it has already been proposed. An example of the bipolar transistor using single crystal silicon/germanium for the base layer is disclosed, for example, in IEDM Technical Digest, 1993, p. 71 to 74. FIG. 2 shows the distribution of germanium in the bipolar transistor in this example. The abscissa denotes the depth in the crystal body while the ordinate denotes the germanium ratio (%). The diagram shows regions for the emitter, base, collector and depletion layer in each of the junction regions. Lines (a), (b), and (c) in FIG. 2 show examples of germanium distribution in these regions.
Generally, by using single crystal silicon/germanium with a smaller band gap than that of single crystal silicon for the base layer of a bipolar transistor, the injection efficiency of electrons from the emitter to the base is improved to increase the collector current and, accordingly, a sufficient current gain can be obtained. Then, as distribution (a) or distribution (b) in FIG. 2 shows, the band gap is decreased from the emitter to the collector to form an electric field in the base layer by increasing the germanium content in the base layer from the emitter to the collector. As a result, electrons in the base layer can be accelerated, and the operation speed of the semiconductor device can be increased. Further, since the collector current is controlled by the electric field in the base layer, it is possible to obtain a good characteristic in which the collector current does not change substantially even if the collector voltage is changed.
The present invention intends to provide a bipolar transistor which is of high reliability and high gain and which is suitable for high speed operation. In view of a more concrete technical feature, the present invention provides a bipolar transistor capable of operating with high accuracy with no substantial change in a collector current even when there is a change in a collector voltage with the bipolar transistor of the present invention also operates with less fluctuation of the collector current, while ensuring high speed operation and high gain.
For attaining such a purpose, problems to be solved will be explained below referring, for example, to a bipolar transistor using a single crystal silicon/germanium for a base layer as a typical example of the present invention.
A bipolar transistor using single crystal silicon/germanium having the germanium distributions (a) and (b) as shown in FIG. 2 described above involves the following problems.
That is, an end position EBb0 of the depletion layer for the emitter/base junction on the side of the base situates at a position where the germanium contents change. Therefore, when the end position EBb0 of the depletion layer for the emitter/base junction on the side of the base changes, the band gap at the position fluctuates. Fluctuation of the band gap at the end position EBb0 is caused, for example, in a case where the depth of the emitter region usually formed by thermal diffusion changes. Since the collector current changes in accordance with an exponential function of the band gap, the collector current changes greatly by the fluctuation of the position EBb0 of the depletion layer for the emitter/base junction on the side of the base.
In order to prevent this, it may be considered to make the distribution of germanium uniform in the base layer, as shown in the distribution (c). However, in this case, it is not possible to obtain a high operation speed obtained by the germanium distribution or a characteristic that the collector current does not change substantially, even when the collector voltage is changed as shown in the distribution (a) or the distribution (b) in FIG. 2.
The present invention intends to provide a semiconductor device, particularly, a bipolar transistor of high accuracy which is capable of attaining a high speed operation or a characteristic that a collector current does not change substantially, even when a collector voltage is changed, and with less fluctuation of the collector current. The present invention is useful, particularly, in a bipolar transistor using a single crystal silicon/germanium layer as a base layer.